SnBiSb Low-Temperature Solder Composition for Brittle Joint Reliability
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Solution Overview
Problem
Current lead-free SnAgCu solder alloys have high melting points, causing temperature sensitivity issues in reflow soldering, and SnBi solder alloys suffer from brittleness and poor reliability due to Bi-rich phase segregation, leading to defects like Head-On-Pillow and Stretched Joint in micro-chip and BGA welding.
Innovation Solution
A SnBiSb-X lead-free solder alloy with a peritectic or near-peritectic structure and fine grains, where Sb and Bi are infinitely solid soluble, reducing stress concentration and improving mechanical performance by eliminating Bi-rich phase segregation, and incorporating additional elements like Ce, Ti, Cu, Ni, Ag, and In for enhanced properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If SnBi series solder is used for low-temperature welding, then the melting point is reduced, but the alloy becomes brittle and forms Bi-rich bands that decrease reliability
Solution Approach 1:
The patent creates a ternary composite solder alloy system (Sn-Bi-Sb) that combines three different metal elements to achieve properties superior to binary SnBi alloys. The specific composition ranges (Sn: 30-60%, Bi: 35-65%, Sb: 0.1-5%) are optimized to form a composite structure where Sb distributes uniformly to prevent Bi-rich phase segregation, thereby maintaining low melting point while improving reliability through enhanced microstructural homogeneity
Solution Approach 2:
The patent systematically varies the compositional parameters of the Sn-Bi-Sb alloy system to optimize performance. By adjusting the ratios of Sn, Bi, and Sb within specific ranges, and controlling the cooling rate (10-100°C/min) and holding time (5-30 minutes) during solidification, the patent achieves a peritectic or near-peritectic structure that eliminates Bi-rich band formation while maintaining low melting temperature
2Reliability
If SnAgCu solder is used for lead-free soldering, then reliability is improved, but the melting point becomes too high causing temperature sensitivity issues
Solution Approach 1:
The patent changes the compositional parameters from traditional SnAgCu to a SnBiSb system with optimized ratios. By controlling the Bi content (35-65%) and Sb content (0.1-5%), the melting point is reduced to 138-160°C while maintaining welding reliability through the formation of a homogeneous peritectic structure that prevents defects
3Temperature
If Bi content is increased to achieve lower melting point, then temperature is reduced, but Bi-rich phase segregation increases causing brittleness
Solution Approach 1:
Sb acts as an intermediary element that mediates between Sn and Bi phases during solidification. The Sb atoms distribute uniformly throughout the alloy matrix and interact with Bi atoms to prevent their segregation into concentrated Bi-rich bands. This intermediary effect maintains composition uniformity even at high Bi content (35-65%), enabling low melting point while preventing brittleness
Solution Approach 2:
The patent achieves uniform local distribution of Sb throughout the alloy matrix, creating consistent local chemistry that prevents Bi-rich phase segregation. The controlled solidification process (cooling rate 10-100°C/min with 5-30 minute holding time) ensures that each local region develops a homogeneous peritectic structure, eliminating the local Bi-rich bands that cause brittleness
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The SnBiSb alloy achieves a significantly lower melting point, improved brittleness, and reliability, addressing the defects in SnBi solder alloys with a peritectic or near-peritectic structure and enhanced mechanical performance, suitable for low-temperature soldering applications.
Implementation Method 1
the Sb element and Bi are infinitely solid soluble, and thus can maximally eliminate the stress concentration caused by 'Sn decrease and Bi increase' during solidification process
Implementation Method 2
The SnBiSb alloy has a peritectic or near-peritectic structure with fine grains
Data Source
AI summary
A SnBiSb series low-temperature lead-free solder and a preparation method thereof, which belongs to the technical field of low-temperature soldering. The lead-free solder includes by weight the following composition: 32.8-56.5% of Bi, 0.7-2.2% of Sb, with the remainder being Sn, wherein the weight percentages of Bi and Sb satisfy a relationship of b=0.006a2−0.672a+19.61+c, wherein the symbol a represents the weight percentage of Bi, the symbol b represents the weight percentage of Sb, and the range of c is −1.85≤c≤1.85. The solder alloy has a peritectic or near peritectic structure with a low melting point, and has an excellent mechanical performance and reliability, and applicable to the field of low-temperature soldering.